1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or the like, and more particularly, to an image forming apparatus including a sheet separation mechanism that separates sheets of a recording medium from a transport member using air.
2. Description of the background Art
Conventionally, a generally known image forming apparatus employs a fixation method using a heating roller. In such a fixation method, heat and pressure are applied to a unfixed toner image on a recording sheet in a nip portion where a pressure roller and a fixing roller including a halogen heater and so forth meet and press against each other while the recording sheet is carried in the nip and transported. Such a fixation method is widely used.
Alternatively, there is another known fixation method, known as a belt fixation method, in which an endless fixing belt is wound around and stretched between the heating roller including the halogen heater or the like and the fixing roller.
In this method, the fixing roller is pressed by a pressure roller through the fixing belt, forming the fixing nip. Heat and pressure are applied to the unfixed toner image on the recording sheet in the nip portion where the pressure roller and the fixing belt meet and press against each other while the recording sheet is transported therebetween.
This configuration allows the heat capacity of the fixing belt to be relatively small so that time for warming up can be reduced, resulting in power saving.
With the foregoing configurations, the toner image fused on the recording sheet contacts the fixing roller/belt. For this reason, the surface of the fixing roller or the fixing belt is coated with a material having good releasability, for example, fluororesin, so as to facilitate separation of the recording sheet from the fixing roller/belt. In addition, in order to physically separate the recording sheet from the fixing roller/belt belt, a separation pawl is employed.
However, a drawback to the use of a separation pawl is that, because the separation pawl contacts the fixing roller/belt, it may easily scratch the surface of the fixing roller/belt, leaving a scratch mark or a trace thereon. When this happens, the output image has undesirable markings such as streaks.
To counteract this possibility, in general, in a monochrome image forming apparatus, the fixing roller consists of a metal roller the surface of which is coated with Teflon in order to make the surface scratch-resistant. Accordingly, the product life of the fixing roller of this kind is relatively long.
The separation claw was used for a relatively long time because it was effective to prevent paper jams due to the recording sheet getting wound around the fixing roller.
However, in a case of a color image forming apparatus, in order to improve color enhancement, the fixing roller includes a surface layer formed of silicone rubber coated with fluorine. In general, a tube made of PFA having a thickness of some tens of microns is used for this purpose. Alternatively, the surface of the silicone rubber is coated with oil.
A drawback of the foregoing configuration is that the surface layer is relatively soft and thus damaged or scratched easily. As described above, when there is a scratch on the surface layer, the output image will have streaks.
In view of this, more recent color image forming apparatuses rarely employ the separation pawl or the like that directly contacts the fixing roller to separate the recording sheet from the fixing roller. Instead, such image forming apparatuses employ a contactless separation method.
However, a drawback of the contactless separation method is that it can cause paper jams when the viscosity of the toner and of the fixing roller is relatively high, causing the recording sheet to roll around the fixing roller after fixation. In particular, when a color image is formed, a plurality of color layers is overlaid on one another, increasing viscosity and thus causing paper jams more easily.
One example of a known separation technique employed in the color image forming apparatus uses a contactless separation plate that extends parallel to the fixing roller/belt in a longitudinal or width direction thereof. A slight gap of approximately 0.2 to 1 mm is provided between the fixing roller/belt and the separation plate.
Another example of known separation technique uses contactless separation pawls aligned with a predetermined interval between each other. A slight gap of approximately 0.2 to 1 mm is also provided between the fixing roller/belt and the separation pawls.
Still another approach is one in which the recording sheet is separated naturally from the fixing roller/belt using the resilience of the recording sheet itself and elasticity of a curved portion of the fixing roller/belt. This technique is a so-called self-stripping method.
In these known separation methods, a gap is provided between the fixing roller/belt and the separation members. Thus, when a relatively thin recording sheet or the recording sheet having a small or no margin at the leading edge is fed, or a solid image such as a photograph is printed, the recording sheet passes through the gap while sticking tightly to the fixing roller/belt, causing the recording sheet to wind around the fixing roller/belt or contact the separation plate and the separation pawls. As a result, paper jams occur.
In view of the foregoing, in order to help the contactless separation devices to separate the recording sheet from the fixing roller/belt, a method is proposed in which air is blown against a sheet separation area such as the nip portion where the pressure roller and the fixing roller meet.
Most air supply mechanisms include a compressor or air pump that compresses air, and air is injected using a solenoid valve that regulates air supply. This configuration allows a relatively large amount of air to be supplied at high pressure.
However, when the compressor is used, the size of the image forming apparatus as a whole increases. In addition, compression of air takes time until a desired high pressure is obtained. Consequently, the compressed air cannot be used immediately after the image forming apparatus is turned on.
Furthermore, the solenoid valve is required, thereby increasing the number of parts and thus significantly increasing the cost of the device. Moreover, when the compressor is driven, causing significant noise, it is not suitable for office use. Such an air supply mechanism tends to be large, consuming a significant amount of power, thereby defeating the purpose of power saving. Finally, the typical image forming apparatus using the compressor is usually a full-color high speed printing machine that tends to be large, expensive, and requiring a dedicated operator.